Enzyme-based biosensors are devices in which an analyte-concentration-dependent biochemical reaction signal is converted into a measurable physical signal, such as an optical or electrical signal. Such biosensors are widely used in the detection of analytes in clinical, environmental, agricultural and biotechnological applications. Analytes that can be measured in clinical assays of fluids of the human body include, for example, glucose, lactate, cholesterol, bilirubin and amino acids. The detection of analytes in biological fluids, such as blood, is important in the diagnosis and the monitoring of many diseases.
Biosensors that detect analytes via electrical signals, such as current (amperometric biosensors) or charge (coulometric biosensors), are of special interest because electron transfer is involved in the biochemical reactions of many important bioanalytes. For example, the reaction of glucose with glucose oxidase involves electron transfer from glucose to the enzyme to produce gluconolactone and reduced enzyme. In an example of an amperometric glucose biosensor, glucose is oxidized by oxygen in the body fluid via a glucose oxidase-catalyzed reaction that generates gluconolactone and hydrogen peroxide, then the hydrogen peroxide is electrooxidized and correlated to the concentration of glucose in the body fluid.
Some biosensors are designed for implantation in a living animal body, such as a mammalian or a human body, merely by way of example. In an implantable amperometric biosensor, the working electrode is typically constructed of a sensing layer, which is in direct contact with the conductive material of the electrode, and a diffusion-limiting membrane layer on top of the sensing layer. The sensing layer typically consists of an enzyme, an optional enzyme stabilizer such as bovine serum albumin (BSA), and a crosslinker that crosslinks the sensing layer components. Alternatively, the sensing layer consists of an enzyme, a polymeric redox mediator, and a crosslinker that crosslinks the sensing layer components, as is the case in—“wired-enzyme” biosensors.
In an implantable amperometric glucose sensor, the membrane is often beneficial or necessary for regulating or limiting the flux of glucose to the sensing layer. By way of explanation, in a glucose sensor without a membrane, the flux of glucose to the sensing layer increases linearly with the concentration of glucose. When all of the glucose arriving at the sensing layer is consumed, the measured output signal is linearly proportional to the flux of glucose and thus to the concentration of glucose. However, when the glucose consumption is limited by the rate of one or more of the chemical or electrochemical reactions in the sensing layer, the measured output signal is no longer controlled by the flux of glucose and is no longer linearly proportional to the flux or concentration of glucose. In this case, only a fraction of the glucose arriving at the sensing layer is contributing to the current. The current no longer increases linearly with the glucose concentration but becomes saturated, meaning that it increases less and less for a given increment of glucose concentration, and eventually stops increasing with the concentration of glucose. In a glucose sensor equipped with a diffusion-limiting membrane, on the other hand, the membrane reduces the flux of glucose to the sensing layer such that the sensor does not become saturated, or becomes saturated only at much higher glucose concentrations and can therefore operate effectively resolve an increase in the concentration of glucose when the glucose concentration is high.
There are various glucose-diffusion-limiting membranes. The membranes are, however, usually made of polymers, and either their average thickness and/or the microscopic uniformity of their thickness is difficult to control and/or reproduce. As a result, the glucose fluxes through the membranes, which determine the sensitivities of the glucose sensors employing such membranes are widely scattered, indicative of lack of adequate control in the membrane-making process. Thus, there is a need for a glucose-diffusion-limiting membrane that provides adequate regulation of the flux of glucose to the sensing layer and that is mechanically strong, biocompatible, and easily and reproducibly manufactured.
In an implantable amperometric glucose or other analyte sensor, the membrane can be also beneficial or necessary for regulating or limiting the flux of an interferent to the sensing layer, the interferent affecting the signal, for example the current produced by the analyte. By affecting the signal, the interferent adds to the measurement's error. The preferred membranes reduce the flux of the interferent more than they reduce the flux of the analyte, for example of glucose.